Process for the production of a light sensitive material for electrophotography

ABSTRACT

A LIGHT SENSITIVE MATERIAL SUITABLE FOR ELECTROPHOTOGRAPHY AND THE METHOD FOR PRODUCING THE MATERIAL. ADMIXED IN AQUEOUS SOLUTION ARE A CADMIUM ION WHICH IS SELECTED FROM THE GROUP CONSISTING OF CADMIUM HALIDE, CADMIUM SULPHATE AND CADMIUM NITRATE; A CARBONATE ION WHICH IS SELECTED FROM THE GROUP CONSISTING OF SODIUM CARBONATE, POTASSIUM CARBONATE AND AMMONIUM CARBONATE; AND A SULFIDE ION WHICH IS SELECTED FROM THE GROUP CONSISTING OF SODIUM SULFIDE, AMMONIUM SULFIDE AND HYDROGEN SULFIDE. FURTHER, A SELENIDE MAY DISPLACE THE SULFIDE ION. A PRECIPITATE IS THUS FORMED WHICH IS THEREAFTER CALCINED AT A TEMPERATURE LESS THAN ABOUT 400*C.

June 29, 1971 IWAO SAWATO ETAL 3,589,928

. PROCESS FOR THE PRODUCTION OF A LIGHT SENSITIVE MATERIAL FORELECTROPHOTOGRAPHY Filed Aug. 4, 1966 VOLTAGE I 2 3 5 7 I0 I00 SEC.

HALF DELAY EXPOSURE TIME INVENTORS. IWAO SAWATO KATSUO MAKINO ATTORNEYS,

United States Patent Int. Cl. otisc 1/00 US. Cl. 11734 6 Claims ABSTRACTOF THE DISCLOSURE A light sensitive material suitable forelectrophotography and the method for producing the material. Admixed inaqueous solution are a cadmium ion which is selected from the groupconsisting of cadmium halide, cadmium sulphate and cadmium nitrate; acarbonate ion which is selected from the group consisting of sodiumcarbonate, potassium carbonate and ammonium carbonate; and a sulfide ionwhich is selected from the group consisting of sodium sulfide, ammoniumsulfide and hydrogen sulfide. Further, a selenide may displace thesulfide ion. A precipitate is thus formed which is thereafter calcinedat a temperature less than about 400 C.

This invention relates to a process for the production of aphotoconductive powder consisting of cadmium sulfide and cadmiumcarbonate and in particular, a light-sensitive material forelectrophotography.

As a light-sensitive material for electrophotography, glassy selenium orzinc oxide has been used. In addition, cadmium sulfide, zinc sulfide,antimony trisulfide, titanium oxide, mercury sulfide or lead oxide maybe used. One of them is applied to a suitable support to form a uniformlayer as in the case of using glassy selenium or dispersed in anelectrically insulating binder to form a thin layer when using zincoxide. The electrophotographic light-sensitive body consisting of auniform layer of glassy selenium has excellent characteristics exceptthat skilled technique is required for the production thereof. On theother hand, the electrophotographic light-sensitive body wherein aphotoconductive fine grain is dispersed in a binder may be produced in arelatively simple manner, but is not generally favorably compared withglassy selenium with respect to characteristics and usage.

The present invention is concerned with a process for the production ofa photoconductive fine powder suitable for use as an electrophotographiclight-sensitive material with a binder.

The characteristics generally required for electrophotographiclight-sensitive materials are that the electrically insulating propertybe large enough so as to maintain electrostatic charges for a long timein a dark place and that the photoconductive property be displayed byradiation of a radiant ray such as visible light to discharge theelectrostatic charges rapidly. Moreover, such characteristics andconditions are preferred that there is a high sensitivity to a radiantray, such as visible light, that is, the electrostatic charges arerapidly discarged by a small quantity of radiation, the characteristicsbeing independent of the state of the material before use. It is chargedelectrostatically positive or negative and exhibits excellent, similarsensitivities to both the polarities, while keeping the cost of thematerial low. With respect to the widely used zinc oxide, the foregoingrequirements and conditions are not necessarily satisfied, as thesensitivity is low, the characteristics are influenced by a state of thematerial before use to a great extent and the light-sensitivity isrestricted to the negative polarity. In order to overcome thesedisadvantages of the zinc oxide, a number of studies have beendisclosed. For example, the sensitivity is increased by colorsensitizing with organic dyes. However, this is accompanied by alowering of the heat stability and a bad influence upon thecharacteristics by the history of the material, in particular, bypre-exposure. On this point too, a number of improvements have beenproposed, but it has been impossible to obtain a material useful withina short period, such as less than 10 seconds.

One object of this invention is to provide a new photoconductive finepowder having a light-sensitivity to either the positive or negativestatic charges and being free of influence upon its characteristics bythe history of the material.

Another object of this invention is to provide a process for producingthe photoconductive fine powder in a simple and stable manner.

A still further object of this invention is to provide an insulatingphotoconductive layer, in particular, a lightsensitive material forelectrophotography by dispersing the photoconductive fine powder in abinder, applying the mixture to a support to form a layer and drying.

A still further object of the invention is to provide a photoconductivepowder consisting of the photoconductive fine powder and a binder.

It has been well known that cadmium sulfide may be used as aphotoconductive material and dispersed in an electrically insulatingbinding agent, may also be used as a light-sensitive material forelectrophotography. However, this material has a high sensitivity tonegative polarity, but not to positive polarity. Furthermore, since theresponse speed of photoconductivity is low and pre-exposure effectremains for several seconds, it is unsuitable for a repeated use withina period of about 10 seconds.

We have found that powder particules consisting of cadmium carbonate andcadmium sulfide give a relatively high response velocity ofphotoelectric current, rather than pure cadmium sulfide fine powder andan electrophotographic light-sensitive body produced by dispersing thepowder in an insulating resin and applying to an electrically conductivesupport has a sufiicient property to be charged in a dark place and highlight sensitivity. Further, it has been found that this light-sensitivebody is well charged positively and negatively and exhibits a similardegree of light sensitivity to both.

A feature of the invention consists in producing a photoconductive finepowder consisting of cadmium sulfide and cadmium carbonate by the wetprecipitating reaction followed by calcination, wherein cadmiumcarbonate is coprecipitated with cadmium sulfide by addition of acarbonate ion.

This wet reaction precipitate may be prepared by mixing an aqueoussolution containing carbonate ion and sulfide ion with an aqueoussolution containing cadmium ion, an aqueous solution containingcarbonate ion with an aqueous solution containing cadmiumion in thepresence of hydrogen sulfide or an aqueous solution containing carbonateion in which hydrogen sulfide is dissolved under pressure with anaqueous solution containing cadmium ion. A small amount of other ionsmay be added to improve the characteristics. The precipitate thusobtained contains no solid solution of sulfide and carbonate as isproved by the X-ray analysis. Although its structure has not yet beenmade clear, it is not a mere mixture of cadmium carbonate powder andcadmium sulfide powder.

As the cadmium salt, cadmium halide, cadmium sulfate or cadmium nitratemay be used. As the carbonate, sodium carbonate, potassium carbonate orammonium carbonate may be used. As the sulfur-containing compound,sodium sulfide, ammonium sulfide or hydrogen sulfide may be used. Sodiumselenide or hydrogen selenide may be added to an aqueous solutioncontaining carbonate ion to displace a part of the sulfur with seleniumso as to control the precipitation.

In the production of a photoconductive fine powder consisting mainly ofcadmium sulfide and cadmium carbonate by the wet process, thecoprecipitation thereof comprising mixing an aqueous solution containingcadmium ion with an aqueous solution containing carbonate ion andsulfide ion is carried out always, but another procedure may be carriedout comprising preparing a fine white precipitate of cadmium carbonatefirstly and then adding a solution containing sulfur so as to convert apart of the white precipitate of cadmium carbonate into cadmium sulfide.In the latter case also, a part or all of the sulfur may be displaced byselenium.

Our invention will now be illustrated by the following examples, but itwill be understood it is not limited thereby.

EXAMPLE 1 142.5 g. of cadmium chloride (CdCl -2 /2H O) was dissolved indistilled water to prepare 1250 ml. of the aqueous solution (B sodium).37.2 g. of sodium carbonate (Na CO -H O) and 15.6 g. of sodium sulfide(Na s) were dissolved in distilled water to prepare 500 ml. of theaqueous solution (A solution). The B solution was added dropwise to theA solution with stirring at room temperature, the dropping velocitybeing about mL/ min, to yield a yellow precipitate. After decanting andfiltering, the precipitate was washed with ethanol and dried in vacuumto yield a dried fine powder. The powder was slightly yellow and had agrain size of 0.1 micron or below. The observation by an electronmicroscope showed that cadmium sulfide and cadmium carbonate appeared tobe one substance intermingled uniformly into a fine grain.

This resulting fine powder was charged to a crucible of quartz, coveredup and calcined in the air at 200 C. for 8 hours. The appearance of thecalcined powder was not changed, as shown by an electron microscope. TheX-ray diffraction showed that the calcined powder gave a diffractionfigure suggesting that it was amorphous, the X-ray diffraction intensityof cadmium sulfide being low and the width being wide as much as that ofthe noncalcined powder. When cadimum sulfide powder is calcined at 200C., for 8 hours, the crystallization to some extent is observed, but nocrystallization occurs in our powder. This means that there is at leastan interaction physical and crystallographical, not mechanical, withcadmium carbonate.

The calcined powder and non-calcined powder were dispersed respectivelyin a binder in a ball mill of porcelain to provide a light-sensitivepaint as shown in Table 1.

TABLE 1 Powder Binder, g.

No. of light Charge, Thinner, sensitive paint Class g. Solid Charge ml.

A Non-calcined 60 21.6 53.2 70 B Calcined 60 21. 6 53. 2 70 Blendingtime in the ball mill: 42 hours.

Binder: A thermo-setting acryl paint, Magicron No. 200 Clear,manufactured by Kansai Paint Co. Ltd.

Thinner: A thinner belonging to the foregoing paint.

tential, the decay of the surface potential during darkening and theattenuation during exposure with respect to each of positive andnegative polarities, after charging them by positive and negative coronadischarge. The exposure was conducted by the use of a tungsten lamphaving a color temperature of 2660 K. and the intensity of illuminationon the surface of the sample was approximately 15 lux. The measurementsshowed that the characteristics in positive and negative charging weresubstantially the same. With the increase of the coating thickness, theinitial surface potential increased and the light sensitivity decreased.That is, the decay velocity during exposure is lowered with an increasein coating thickness. The decay during darkening was hardly changed bythe coating thickness. The characteristics were improved by baking,remarkably in the non-calcined powder, but not so in the calcinedpowder.

In FIG. 1 there is shown a relation of the initial surface potential tothe half decay exposure time (time required for half-value of thesurface potential to decay exposure) as to samples ofelectrophotographic lightsensitive body which are prepared by the use ofthe lightsensitive paint A or B under a baking condition of 250 C. at 30minutes. In this figure, the more off the curve to the left the betterthe characteristics. It is evident from this that the calcined powdercompares favorably with the non-calcined powder and the positivecharging and negative charging give substantially similar characteristics in each of the calcined and non-calcined powder.

EXAMPLE 2 The calcined powder of Example 1 was mixed with the binderused in Example 1 in a proportion of 70 g. of the former to 15 g. of thelatter and dispersed adequately. The mixture was dried by hot air at6080 C. with vigorous stirring. The dried mixture was in small cake-likeforms which had tho be finely divided. The dried, small blocks, weresubjected to heat treatment at C., for 60 minutes to harden the bindercompletely, and charged into a ball mill of porcelain with ethylalcohol, finely pulverized and then the ethyl alcohol was evaporated toyield a fine dry powder. This powder was then spread uniformly on anelectrically conductive support, charged uniformly by corona dischargeand exposed to light. The discharging of the charges on the powderoccurred selectively according to the intensity of illumination. Whenapplying an air jet to the surface, an image according to the intensityof illumination of the exposure was obtained. The light sensitivity wassomewhat unfavorable when compared with Example 1 but substantially thesame to either the positive or negative polarities.

EXAMPLE 3 A solution was prepared by dissolving 172.1 g. of cadmiumbromide (CdBr -4H O) in distilled water to 1000 ml. of solution and Bsolution was prepared by dissolving 37.2 g. of sodium carbonate (Na CO-H O) and 15.6 g. of sodium sulfide (Na S) in distilled water to make500 ml. of solution. The A solution was added dropwise to the B solutionwith stirring to yield a yellow precipitate and the resultingprecipitate was fired at 200 C., for 6 hours in the air to give aphotoconductive powder. The subsequent blending with an organic binderand treatment were carried out similarly as in Example 1.

EXAMPLE 4 A solution was prepared by dissolving 128.3 g. of cadmiumcarbonate (CdCO -8/3H O) in distilled water to make 1000 ml. of solutionand B solution was prepared by dissolving 34.3 .g. of ammonium carbonateand 13.6 of ammonium sulfide in distilled water to make 1000 ml. ofsolution. The subsequent procedures and treatments were similar toExample 3.

EXAMPLE 5 A solution was prepared by dissolving 142.5 g. of cadmiumchloride (CdCl -2 /2H O) in distilled water to make 1000 ml. of solutionand B solution was prepared by dissolving 37.2 g. of sodium carbonate(Na CO -H O), 9.8 g. of sodium sulfide and 11.0 g. of sodium selenide(Na Se) in distilled water to make 1000 ml. of solution. The subsequentprocedures and treatments were similar to, Example 3.

EXAMPLE 6 A light-sensitive paint was prepared from the calcined powderof Example 1 according to the following mixing ratio and applied to analuminum plate, followed by drying to produce a sample ofelectrophotographic lightsensitive body. The drying was carried out at50 C. for 2 hours. No baking was done.

Calcined powder of Example 1-88 g.

Silicone resin KR-211 (made by Shin-Etsu Chemical Co.)-22.9 g.

Epoxy ester D-4 (made by Nippon Oil & Fat Co.)-

Toluene-50 ml.

The blending was carried out in a ball mill of porcelain for 42 hoursand the binder was capable of being dried at normal temperature. Themeasurement of the electrophotographic characteristics under a similarcondition to Example 1 gave substantially the same results as in Example1.

EXAMPLE 7 100 g. of the calcined photoconductive fine powder obtained inExample 1 was mixed with g. of Beckacite as a solvent and the mixturewas dried in vacuum to yield a porous cake. The resulting cake wasfinely divided to give a powder consisting of the binder andphotoconductive fine powder, which was suitable for use as aphotoconductive toner for electrophotography. That was fused and fixedby means of a solvent such as trichrene or by heating.

EXAMPLE 8 A solution was prepared by dissolving 212 g. of anhydroussodium carbonate in distilled water to make 1.5 of solution. B solutionwas prepared by dissolving 513 g. of cadmium sulfate (CdSO -8/3H O) indistilled water to make 1.5 1., and C solution was prepared bydissolving 78 g. of anhydrous sodium sulfide in distilled water to make0.3 l. of solution. The B solution was dropwise added to the A solutionwith stirring to yield a white precipitate of cadmium carbonate, towhich the C solution was then added dropwise to convert a part of thecadmium carbonate into cadmium sulfide. The thus obtained yellowprecipitate was washed with water, completely dried and fired in the airat 200 C. for 24 hours to give a photoconductive powder. The powder wasmixed with a binder in the similar manner to Example 1 to obtain asample of light-sensitive body for electrophotography.

It will be understood that the precipitate formed by mixing an aqueoussolution containing cadmium ion with an aqueous solution containingsulfide ion and carbonate ion has excellent photconductive propertiesand is further improved by calcination. The addition of other ions ordyes during precipitating, after precipitating or before calcining beemployed for the purpose of improving the characteristics as anapplicable variation of our invention. The calcination or firing is notalways carried out in the air, but may be in oxygen gas or inert gas inaccordance with the desired characteristics. The calcination may beconducted to such an extent as to decompose the formed cadmiumcarbonate, but preferably at a temperature below 400 C.

Since the photoconductive fine powder of our invention turns thinyellow, in a mixed disperlsion system, to the photoconductive finepowder and binder, may be added other inorganic orv organic pigments forimproving or coloring the reflected ray, and dyes for color sensitizing.

What is claimed is:

1. A process for producing photoconductive fine powder which comprisesadmixing in aqueous solution cadmium ions, carbonate ions and sulfideand/or selenide ions, said materials admixed in a molar ratio of about22 to about mol percent cadmium ions, from about 15 to about 57 molpercent carbonate ions and about 6 to about 40 mol percent of sulfideand/or selenide ions whereby a precipitate is formed, and thereaftercalcining the precipitate at a temperature of less than about 400 C.

2. A process for producing a photoconductive element wherein thecalcined precipitate of claim 1 is dispersed into an electricallyinsulating binder to form a liquid dispersion, applying the dispersionto a suitable support and drying.

3. A process for producing a photoconductive fine powder as in claim 2wherein said calcining is carried'out after said drying.

4. A process as in claim 1 where said cadmium ion is selected from thegroup consisting of cadmium halides, cadmium sulphate and cadmiumnitrate, said carbonate ion is selected from the group consisting ofsodium carbonate, potassium carbonate and ammonium carbonate, and saidsulfide ion is selected from the group consisting of sodium sulfide,ammonium sulfide and hydrogen sulfide.

5. A process as in claim 1 where said calcining occurs at 200 C.

6. A product manufactured in accordance with the process of claim 1.

References Cited UNITED STATES PATENTS 2,496,587 2/1950 Marcot 23-134NORMAN E. TORCHIN, Primary Examiner J. R. HIGHTOWER, Assistant ExaminerU .3. Cl. X.R. 252-501; 96-15

